In the field of adhesive bonding there is a substantial body of literature devoted to improving adhesion of various adhesive systems. The various techniques include altering substrate surfaces by cleaning, priming with intermediate adhesive materials, irradiating the surfaces, etc.; subjecting curable adhesives to specifically sequenced cure cycles; priming the substrate surfaces with various cure initiators or accelerators; and altering the adhesive composition by including specific compounds which improve the bond strength of the adhesive to the bonded substrates. Examples of the compounds in the latter category are silane compounds used in RTV silicone formulations which contain both polar groups compatible with metallic surfaces and hydrolyzable groups; and acrylic, maleic or fumaric compounds with various acid functionalities as well as silane compounds with acrylic and hydrolyzable groups, all of which have been used in acrylic adhesives of various types. Other examples are well known in the various curable adhesive arts.
The utility of co-curable adhesion promoters, however, tends to be very system specific. The particular adhesion promoter must not only provide functionalities which effectively provide a bridge from the adhesive to the substrate surface, it must also not significantly deteriorate the performance of the adhesive. Consequently, the selection of adhesion promoting additives in new curable systems cannot be made on the basis of performance in another curable system.
Thiolene curable systems involve addition of thiol (--SH) groups across an olefinic or acetylenic double bond. The reaction may be catalyzed by acids or free radicals. A detailed discussion of the mechanism of this reaction and of the academic literature in this field may be found in Oae, ed., "Organic Chemistry of Sulfur", Plenum Press New York, New York, pp. 131-187 (1977). Oswald et al reportedly disclosed photocrosslinkable compositions employing tetraallyl ester compounds and dithiols at the April 1965 ACS meeting in Detroit, Michigan. See Oswald et al, Die Makromolekulare Chemie, 97, 258-266 (1966). Formulations of di-or polythiols and dienes or polyenes which are curable to solids by chemical or photolytic radical generators are also described in U.S. Pat. Nos. 2,767,156; 3,661,744; 3,240,844; 4,119,617; and 4,157,421. All of these prior art curable formulations have involved polythiol additions to double bonds of linear olefins or acetylene compounds.
Silicone polymers having alkyl thiol groups are disclosed in U.S. Pat. No. 4,289,867 and background references discussed therein. In U.S. Pat. No. 4,284,539 other silicones having linear and cyclic mercaptoalkyl groups are described as are the thiol-ene reactions of such materials with silicones having vinyl or silacyclopentene groups. Peroxide cured compositions of vinyl terminated silicones with polythiols having organic or silicone are described in U.S. Pat. No.3,445,419.
In copending application Ser. No. 099,676, filed Sept. 21, 1987, now U.S. Pat. No. 4,808,638, and incorporated herein by reference, there are described improved thiolene curable systems based on compounds having a plurality of norbornenyl or analogous bicyclic ene groups.
Despite the extensive literature on thiolene compositions applicants have been unable to locate any reference which teaches adhesion promoting techniques for such compositions and in particular, have been unable to locate any references disclosing adhesion promoting co-curable additives for thiolene compositions.
In U.S. Pat. No. 4,435,497 there are described polymerizable thiolene resist compositions in which the ene compound contains at least one each of (meth)acryloxy groups; allyl, methallyl, or 1-propenyl groups bonded through an oxygen or oxycarbonyl group to an aromatic or cycloaliphatic nucleus; and carboxylic acid groups. The function of the carboxylic acid group is understood as a latent debonding agent, allowing removal of uncured formulation with a mild base wash and removal of the cured resist with a strong base wash.